Body conformable 915 MHz microstrip array applicators for largesurface area hyperthermia

The optimal treatment with hyperthermia of superficially located tumors which involve large surface areas requires applicators which can physically conform to body contours, and locally alter their power deposition patterns to adjust for nonuniform temperature caused by tissue inhomogeneities and blood flow variations. A series of 915 MHz microstrip array applicators satisfying these criteria have been developed and clinically tested. Clinical and engineering design tradeoffs for practical devices are discussed. Measurements taken in tissue equivalent phantoms and a summary of our clinical experiences with these microstrip arrays are presented.     

Aberrant Heating: A Problem in Regional Hyperthermia

A 180 cell block model of man has been used to compute the pattern of energy deposition when radio-frequency applicators are used for treatment of cancer by hyperthermia. When the abdomen is exposed with polarization parallel to the length of the body at frequencies from 10 to 60 MHz, approximately 60-70 percent of the total energy is deposited outside of the abdomen. This fraction is increased to as much as 90 percent at several resonances which occur between 100 and 500 MHz. The local rates of energy deposition in the arms, neck, chest, and thighs often exceed that in the abdomen. The pattern of this aberrant heating is dependent upon the positions of the arms and legs. Aberrant heating appears to be much less pronounced for treatment of the thigh or upper arm than it is for treatment of the abdomen.     

Em Local Heating with HF Electric Fields (Comments)

The above paper recently described a synthesis procedure that is intended for use in designing multisection capacitor-plate applicators for treatment by hyperthermia. The distribution of potentials on the subelectrodes is determined in order to obtain a specified pattern of energy deposition. While the derivation in that paper appears to be sound, considerable care must be used if the procedure is to be applied for the design of a practical applicator for use in patient treatment.     

EM Local Heating with HF Electric Fields

Capacitor-plate applicators consisting of a pair of flat-plate electrodes, energized by a HP voltage, are utilized to induce heating inside a biological body for the purpose of hyperthermia cancer-therapy. In this paper, a theoretical analysis for such applicators is presented. A pair of coupled integral equations for the unknown total induced electric field inside the body and the charge density on the electrodes is derived and solved numerically for several different cases. The distribution of the specific absorption rate (SAR) of energy inside the body is obtained for each case. The body-electrode coupling is taken into account. A theoretical scheme for synthesizing proper potential distributions on two arrays of subelectrodes for inducing a desirable SAR ditribution inside the body is also developed. Using such arrays of subelectrodes, the excessive heating at the fat layer of the body may be avoided. An experiment has been conducted to test the theory.     

Direct computation of ultrasound phased-array driving signals froma specified temperature distribution for hyperthermia

This paper presents a new method which obtains ultrasound hyperthermia applicator phased-array element driving signals from a desired temperature distribution. The approach combines a technique which computes array element driving signals from focal point locations and intensities with a new technique which calculates focal point locations and power deposition values from temperature requirements. Temperature specifications appear here as upper and lower bounds within the tumor volume, and a focal point placement algorithm chooses focal patterns capable of achieving the temperature range objective. The linear algebraic structure of the method allows rapid calculation of both the phased-array driving signals and an approximate temperature field response. Computer simulations verify the method with a spherical section array (SSA) for a variety of temperature specifications and blood perfusion values. This scheme, which applies to any phased-array geometry, completes an essential step in both treatment planning and feedback for hyperthermia with ultrasound phased-array applicators.     

Body Conformal Antennas for Superficial Hyperthermia: The Impact of Bending Contact Flexible Microstrip Applicators on Their Electromagnetic Behavior

Hyperthermia is a powerful radiosensitizer for treatment of superficial tumors. This requires body conformal antennas with a power distribution as homogeneous as possible over the skin area. The contact flexible microstrip applicators (CFMA) operating at 434 MHz exist in several sizes, including the large size 3H and 5H. This paper investigates the behavior of the electromagnetic fields for the 3H and 5H CFMA in both flat and curved configurations, and the impact on performance parameters like the penetration depth (PD) and the effective heating depth (EHD). The underlying theory behind the electromagnetic behavior in curved situations is presented as well as numerical simulations of both flat and curved configurations. The results are compared to measurements of the electromagnetic field distributions in a cylindrical patient model. Due to their large size multimode solutions may exist, and our results confirm their existence. These multimode solutions affect both the power distribution and PD/EHD, with a dependence on applicator curvature. Therefore, the performance parameters like PD and EHD need to be carefully assessed when bending large size CFMA applicators to conform to the patient body. This conclusion also holds for other types of large size surface current applicators.      

The use of a current sheet applicator array for superficialhyperthermia: incoherent versus coherent operation

There are a number of potential advantages to be gained by using an array of applicators in hyperthermia treatments compared with single applicator systems, These advantages include the possibility of greater spatial control of power deposition and conformability to nonplanar sites. Arrays of applicators can be driven either coherently or incoherently. In the case of coherent operation, an added advantage is the ability to steer power deposition by varying the phases of the antennas. In this study, the authors investigated the relative merits of the two modes of operation when a 2×2 planar array of current sheet applicators is used. The effective field size (EFS) of the array was calculated using a Gaussian beam representation of the applicators on a layered model in which the fat layer had its thickness varied. Good agreement was obtained between the square of the electric field distribution (E²) and quantitative experimental results. It is shown that when the planar array is used with a fat layer greater than about 2 mm present, it should be driven incoherently as this results in a significantly larger EFS than that obtained when the array is driven coherently     

A new coaxial TEM radiofrequency/microwave applicator for non-invasive deep-body hyperthermia

At the moment great efforts are being made to develop non-invasive heating systems which produce controlled local or regional deep-body hyperthermia. Electromagnetic interference techniques (10-80 MHz) with several separated applicators or with multiple applicators can produce deep-body heating. In our institute a coaxial frequency-independent TEM radiofrequency/microwave applicator has been designed. This applicator can produce a theoretically optimal interference maximum in the centre of the body. The applicator is very simple to construct and inexpensive. To test the design with the equipment available, a scaled prototype of the TEM applicator has been developed. The prototype has a diameter of 20 cm and operates at 434 MHz. The applicator has been tested on several phantom materials. The measured absorbed power distributions are in good agreement with the calculated theoretical distributions. The theoretical calculations of the absorbed power distributions of a 10-80 MHz TEM deep-body applicator are very encouraging.     

Optimal power deposition with finite-sized, planar hyperthermiaapplicator arrays

Improved hyperthermia applicator technology is allowing finer spatial power resolution within the heated tissue volume. Effective utilization of these planar applicator arrays requires an understanding of the interrelationships between the lateral dimensions of the tumor and the applicators, the power field produced by the applicators, the amount of surface cooling, the tumor tissue blood perfusion, and the normal tissue blood perfusion. These interrelationships are investigated using three-dimensional power patterns and temperature fields produced by optimizing the power amplitudes of the individual applicators located within an array of small, but finite, planar applicators. Five major conclusions are obtained. First, optimization works and is effective in determining optimal power fields. Second, for optimal treatments the lateral dimensions of a single superficial applicator need to extend beyond the tumor boundary. Third, surface cooling is needed to reduce the high normal tissue temperatures at shallow depths. Fourth, finer power resolution becomes more important as the tumor size decreases, but, little improvement in the temperature field is achieved beyond a 3 x 3 array configuration. Fifth, increasing the normal blood perfusion rate can decrease the temperature on the tumor boundary if direct power deposition on that boundary is unavailable.     

A Model for Impedance Determinations and Power Deposition Characterization in Three-Electrode Configurations for Capacitive Radio Frequency Hyperthermia?Part B: Current Flow and Power Deposition

The 2-D rectangular model for three-electrode radio frequency hyperthermia applicators established in Part A is applied to the determination of the current lines and the potential lines inside the system. Any electrode voltage distributions can be considered. Two figures illustrate the impact of the electric and the geometric parameters on these lines. They show that important changes in local current density, in electric field strength, and in electric field orientation are tied to the adjustments of the relevant parameters. Two successive sampling processes lead to the determination of the local power deposition. The figures which are displayed demonstrate the ability to move this deposition inside any useful area by means of proper trimming of the electrode voltage phases and magnitudes.     

A Whole Body Thenmal Model of Man During Hyperthermia

A whole body thermal model of man has been developed to predict the changes in regional temperatures and blood flows during hyperthermia treatments with the miniannular phased array (MAPA) and annular phased array (APA) applicators. A model of the thermo-regulatory response to regional heating based on the experimental and numerical studies of others has been incorporated into this study. Experimentally obtained energy deposition patterns within a human leg exposed to the MAPA were input into the model and the results were compared to those based upon a theoretical deposition pattern. Exposure of the abdomen to the APA was modeled with and without the aberrant energy deposition that has been described previously. Results of the model reveal that therapeutic heating (42°C) of extremity soft tissue sarcomas is possible without significant systemic heating. Very high bone temperatures (50°C) were obtained when the experimental absorption pattern was used. Calculations show that systemic heating due to APA exposure is reduced via evaporative spray cooling techniques coupled with high-velocity ambient air flow.     

Optimization of Simulated Two-Dimensional Temperature Distributions Induced by Multiple Electromagnetic Applicators

A computer simulation to calculate transient temperature distributions in realistic cross sections of the human body is described. In the simulation, the therapeutic effect of hyperthermia administration is maximized by optimum use of the electromagnetic applicators. The time-dependent mathematical optimum for the levels of energy going to the applicators as well as for the waterbolus separating the applicators from the patients is computed. Among other applications, this modeling is useful for predicting limitations to certain types of hypertherrnia systems, and has the potential for realistic computer-aided therapy planning. As an example, the treatment planning for a tumor in the lower limb is given in the case of incoherent sources.     

Ultrasound applicators with internal water-cooling for high-powered interstitial thermal therapy

Internal water-cooling of direct-coupled ultrasound (US) applicators for interstitial thermal therapy (hyperthermia and coagulative thermal therapy) was investigated. Implantable applicators were constructed using tubular US sources (360 angular acoustic emittance, approximately 7 MHz) of 10 mm length and 1.5, 1.8, 2.2, and 2.5 mm outer diameter (OD). Directional applicators were also constructed using 2.2 mm OD tubes sectored to provide active acoustic sectors of 90 degrees and 200 degrees. A water-cooling mechanism was integrated within the inner lumen of the applicator to remove heat from the inner transducer surface. High levels of convective heat transfer (2100-3800 W/m2K) were measured for practical water flow rates of 20-80 mL/min. Comparative acoustic measurements demonstrated that internal water-cooling did not significantly degrade the acoustic intensity or beam distribution of the US transducers. Water-cooling allowed substantially higher levels of applied electrical power (> 45 W) than previous designs (with air-cooling or no cooling), without detriment to the applicators. High-temperature heating trials performed with these applicators in vivo (porcine liver and thigh muscle) and in vitro (bovine liver) showed improved thermal penetration and coagulation. Radial depth of coagulation from the applicator surface ranged from 12 to 20 mm for 1-5 min of sonication with 28-W applied power. Higher powers (41 W) demonstrated increased coagulation depths (approximately 9 mm) at shorter times (15 s). Thermal lesion dimensions (angular and axial expanse) produced with directional applicators were controlled and directed, and corresponded to the active zone of the transducer. These characteristic lesion shapes were also generally unchanged with different sonication times and power, and were found to be consistent with previous coagulation studies using air-cooled applicators. The implementation of water-cooling is a significant advance for the application of ultrasound interstitial thermal therapy (USITT), providing greater treatment volumes, shorter treatment times, and the potential for treatment of highly perfused tissue with shaped lesions.     

Simulations of MAPA and APA heating using a whole body thermalmodel

Simulations of hyperthermia treatments to the extremities and the pelvis with miniannular phased array (MAPA) and annular phased array (APA) applicators have been conducted using a whole-body thermal model of man. The model is an enhanced version of a simpler model and accounts for gross spatial variations in arterial and venous blood temperatures throughout the body during a hyperthermia treatment. Included in the modified model are constitutive relations governing the local thermoregulatory changes in skin and muscle blood flow and sweating during local heating. Results of these simulations reveal that systemic heating is not significant during extremity heating with a MAPA due to the lack of aberrant energy deposition outside of the treated area. A nonthermoregulated tumor can be preferentially heated to therapeutic levels by the MAPA if it is positioned within the central region of the applicator. Simulations of APA treatments show that systemic heating is quite significant when aberrant electromagnetic energy deposition is taken into account. Comparison of simulated results to clinical data indicates that the modified model can predict the deep body temperatures and cardiac output changes in a more realistic manner than the original model     

Preclinical evaluation of submillimeter diameter microwave interstitial hyperthermia applicators

Ultra miniature coaxial cable has been used, with microscopic techniques, to fabricate interstitial hyperthermia applicators having diameters of 0.20 mm, 0.33 mm, and 0.58 mm; commercial applicators have a diameter of 1.1 mm. Animal studies with the 0.33 mm diameter applicators have shown that they produce less local tissue trauma than the larger-diameter devices. All of these applicators operate at 915 MHz and have similar heating patterns because they use the conventional monopole design and the catheters have been approximately scaled to the dimensions of each size applicator. We have measured the heating (SAR) patterns of these applicators in tissue-simulating phantoms, both singly and in arrays, using a miniature electric field probe. As an intermediate step to patient trials, we have examined the ability of these applicators to provide effective heating of perfused tissue, using pig thigh and liver as models. Test results suggest that the durability and power handling capabilities of our submillimeter applicators are adequate for use in patients. These new applicators should be useful in the percutaneous treatment of deep-seated tumors and in intraoperative treatments. The applicators also permit intraluminal or intravascular access to tumors.     

Numerical and experimental analysis of leaky-wave microwave applicators

The theory of leaky-wave antennas, suitably modified, is applied for the evaluation of the field intensity induced in a lossy medium by two applicator schemes, the grounded dielectric layer and the channel guide. The comparison of calculated and measured results confirm the effectiveness of the leaky-wave approach for the analysis of such microwave power applicators.     

CAD of efficient TMmn0 single-mode elliptical applicators with coaxial excitation.

This paper suggests computational and engineering approaches to designing single-mode elliptical applicators for thermal processing of cylindrical samples of relatively small diameters. Through a systematic computational experimentation employing a 3D conformal FDTD model, we show that stable and efficient excitation of even and odd TMo modes of elliptical waveguides can be achieved by appropriate placing of two (one active and one passive) coaxial probes extended in the longitudinal direction of the cavity of relatively small height and by choosing the dimensions of the cavity. Effects produced by cylindrical loads inserted in the electric field maxima of the applicators featuring even TM010, TM110, and TM210 modes are studied for materials with different dielectric constants and the loss factors. It is shown that with the proposed excitation supported by simulation of energy coupling and dissipated power the elliptical structure is capable of operating any of these modes in a single-mode regime and generating satisfactory uniformity of the dissipated power within the cylindrical samples.     

Sustaining long linear uniform plasmas with microwaves using aleaky-wave (troughguide) field applicator

Linear, uniform, high density plasma sources can be realized using linear microwave field applicators. Such systems can be applied to process large surfaces of materials that can be made to move transversely across the plasma source. This paper details the design and operating characteristics of appropriate linear applicators that are of the leaky-wave type. A general analysis of longitudinally uniform leaky-wave-sustained discharges provides us with the generic features of the corresponding field applicators. On practical ground, we turn to troughguide structures which are well known in the field of antenna engineering; we consider periodically asymmetric troughguides terminated with a matched load and show experimentally that they can be designed to achieve the required linear uniform discharge. To assure the operation of the applicator at microwave powers of the order of one kW or more, we have devised and tested a new kind of ridge-waveguide mode transformers allowing one to feed microwave power to the applicator directly from a standard rectangular waveguide. The main advantages of the leaky-wave applicators are their relative simplicity of design and operation (no tuning elements), a good power match with the microwave generator and a good longitudinal uniformity of the sustained plasma     

A comparison of deep-heating electrode concepts for hyperthermia

There is mounting evidence that localized hyperthermia produced by electromagnetic waves may be useful in the treatment of cancer, and many innovative devices have been designed for this purpose. Most applicators employed for deep heating operate in the frequency region of 10-100MHz to provide greatest depth of penetration. Two basic categories of launching devices exist: E-field and H-field. The E-field applicators include conductive plates and fringing field devices; either may be used individually or in a multiple feed system. The H-field applicators include cylindrical and planar devices configured to produce specific heating patterns. We have analyzed and compared the performance of each of these devices, particularly in terms of engineering principles, design characteristics and their ability to transfer potentially therapeutic energy safely and at depth.     

Experimental characterization of helical coils as hyperthermiaapplicators

A series of helical-coil hyperthermia applicators have been designed for treating human limbs. Several experiments to determine their operating characteristics were conducted using muscle-equivalent, cylindrical, and lower-body-shaped phantoms. It was found that this kind of applicator has to be operated at resonances which are both sharp and load-dependent. This can have significant clinical implications, since changes in the position of the patient and/or the tissue dielectric properties with temperature can produce a severe mismatch. Moreover, even though the patterns of energy deposition were found to be relative transversely uniform and axially belt-shaped within the cylindrical phantoms, they were strongly dependent on the shape of the phantom and of the coil for the more realistic human-shaped phantom. Intense local heating was observed whenever the winding of the helical coil was within a few millimeters of the surface of the human-shaped phantom. The tests with the human-shaped phantom showed that there can be significant energy deposition outside of the region intended for treatment